I've done a few upgrades to a couple of my shotguns recently, and since
I couldn't find any good instructions/pictures/tutorials online, I
thought I'd do a few of my own. Maybe the next guy who is looking for
some installation info will find this helpful.
This, the first tutorial, will cover how to install a Wide Action
Release from Tank's Rifle
Shop in a 12ga Remington 870.

I
can't/won't be held responsible if you try this mod and screw up your
gun. If you're not comfortable working with small tools, please have a
gunsmith do the install for you!

What you get:

The Wide Action Release currently costs $33.40 directly from Tank's.
You will also need to order a replacement hammer pin for your trigger
group. Hammer pins are also available from Tank's for $3.00, but you
will have to make sure you get the one appropriate for your trigger
housing as the metal housing and plastic housing require different
hammer pins.

What it does:

Some people will claim that the Wide Action Release is an answer in
search of a problem, but anyone who operates their shotgun in a
stressful environment (or with gloved hands) will appreciate the larger
surface area of the Tank's release. It is definitely hard to miss as it
increases the surface area of the standard release by three times.
I couldn't determine how the release was modified, but it actually
looks to be a Remington factory release with material added. It's hard
to tell if it is metal or some sort of polymer, but it feels very
solid.

Getting started:

I forgot to take pictures of how to remove the two pins that retain the
trigger group in a Remington 870 but, honestly, if you can't figure
that out on your own, you probably shouldn't be attempting this
modification by yourself.
Once the trigger group is out, the first step is going to be grinding
out the staked end of the stock hammer pin (A).

Just grind down far enough to remove the staking of the pin.

The next step is to drive the hammer pin out, remove the shell carrier
(You can do this before grinding the hammer pin as well) and
disassemble what we can. Pay special attention to the hammer spring and
plunger (A)
and the carrier spring and plunger (B)
as they can go
flying when you start removing other parts (Ask me how I know).

Note:
The hammer spring and plunger are captured by a tab on the action
release. Also note how the bent wire spring is oriented UNDER the
action release.

Once you've got it apart, you should end up with something like this...

Modifying the trigger housing:

This step is pretty simple. Since the Tank's action release is a LOT
wider than the normal one, it won't fit in the slot in the trigger
guard.

Grab your dremel and a cut-off wheel...

...and cut at both ends of the slot...

Since my trigger guard was of the plastic variety, it took about 15
seconds with the cut-off wheel to remove what needed to be removed. If
your trigger guard is metal, it'll probably take a little longer, but
the operation is the same. Clean-up the edges with a file and, if you
have a metal trigger guard you can paint (or otherwise re-blacken) the
exposed metal before putting everything back together.

Reassembly:
Clean all the metal and plastic shavings off of the trigger group
and housing before you start to reassemble!

Putting it all back together is pretty easy. Just follow the
disassembly instructions in reverse using your new hammer pin. Make
sure the bent wire spring is UNDER the action release tab, and that the
same tab captures the hammer spring and plunger. It really helps to
have three hands when you're putting it back together.

Once it is all back together you'll need to stake the new hammer pin. I
used a small flat-head screwdriver and a hammer to stake mine in three
places. The following picture is blurry, but you get the idea.

And here are some more pictures for your viewing pleasure...

Big props to my Dad who was in town for the weekend and helped with the
"gun projects" and picture taking! Sometimes you just need that third
(or fourth) hand and second set of eyes to make sure things go back
together with no leftover bits.